Telephone communications systems that use analog trunks are sensitive to echoes caused by impedance mismatches between a central office (CO) and a CO interface card of the respective telephone system. Echoes, which generally occur in analog or time-division multiplexed (TDM) systems, are created when a user's voice signal is reflected back from another point within the transmission path. These echoes are often caused by an impedance mismatch at a trunk card containing a hybrid interface where 2-wire and 4-wire connection facilities meet. The degree of such impedance mismatch is often called transhybrid balance. As the impedance mismatch increases, the echoes become louder and, conversely, as the impedance mismatch decreases, the echoes become fainter.
In Voice over Internet Protocol (VoIP) applications using analog trunks, the echoes are further enhanced by transmission delays and faulty echo cancellers. Thus, the CO interface cards in VoIP systems should have better impedance-matching characteristics than the interface cards used in pure analog or TDM systems. The superior transhybrid balance helps to reduce the echoes that may appear prior to the convergence of the echo canceller of the respective system.
Several techniques have been proposed to reduce echoes in analog, TDM, or VoIP connections. In one such known technique, test signals are sent from a trunk card to the CO through one or more analog trunks and the results are interpreted to characterize the impedance at the CO and, subsequently, to reduce potential echoes. However, this technique requires the use of expensive and sophisticated equipment.